SUIVilVlARY: Addiction to opioid drugs sucli as morphine is a major public health concern. The complex pathology of opioid addiction can be initiated by activation of specific drug targets in the brain, and the main target of abused drugs is the mu- opioid receptor (MOR), a member of the G protein-coupled receptor (GPCR) family. Activation of GPCRs elicits a sequence of events that results in regulated receptor removal from the cell surface by endocytosis. In the case of MOR, receptor endocytosis controls the de-sensitization and re-sensitization of the neuronal response to MOR signaling, and affects the long-term cellular changes that lead to the development of drug tolerance and dependence. While the traditional view is that regulation of receptor endocytosis is achieved by controlling receptor interaction with the endocytic machinery, my recent studies have identified a novel mechanism by which GPCRs, including MOR, specifically modulate their own endocytosis by controlling the local endocytic machinery. This suggests a novel and unanticipated facet of opioid regulation. The proposed studies seek to identify the mechanistic basis of this regulation and to investigate its functional significance to the effects of clinically relevant opioid drugs. Specifically, this proposal aims to: 1) identify and refine the structural determinants on MOR that mediate regulation of the endocytic machinery;2) establish its mechanistic basis by identifying endocytic regulatory proteins;3) determine the effect of different opioid drugs on this regulation in physiologically relevant neurons;and 4) define the functional consequences of this regulation on MOR signaling. CANDIDATE: The applicant has prior training in cell biology and biochemistry, and is committed to pursuing independent research in the cellular neurobiology of neuropsychiatric disorders and drug addiction. In the K99 phase, he has been mentored by Dr. Mark von Zastrow in the pharmacology, molecular genetics, and neurobiology of signaling receptors implicated in these disorders,